When I was in high school back in the 1970s, I was taught that to make a plural of an acronym, a letter, or a number, one should add an apostrophe and "s". Like I would have written this sentence, "… back in the 1970's …" I would write "one CD, two CD's". Etc. I followed this rule faithfully for years until a co-worker told me it was wrong. Now I can't find any source that agrees with what I was taught.
Is this a rule that has changed over time? Was the convention in the 70s that one should use an apostrophe but this has changed and now we do not? Or were my high school English teachers just confused?
Best Answer
I seem to remember the old askoxford.com site said either was acceptable: CDs and CD's.
But now the replacement Lexico powered by Oxford firmly suggests to avoid the apostrophe except in a few special cases: